Manufacture and use of fluorescent lead sulphate materials



Julyv14,"1942. F. F. RENWICK 2,289,384

MANUFACTUBE AND USE OF FLURSGENT LEAD SULPHATE MATERIALS- Filed Feb. 12,194C]A Y Euppnrl Fig PTE.

Laad Sulfaa E E ns v e Emulsun Emp p n FT 5e nsifive Emneinn Thin SuppE11-T 2'/ Laad EQUIELTE 'Q1-bm gmc/Wto@ Rim-lk F31" STER Eewiuk @M firmaPatented July 14, 1942 MANUFACTURE` AND USE 0F FLURESCENT LEAD SULPHATE'MATERIALS Frank Forster Renwick, Ilford, England, assignor to IlfordLimited, Ilford, Essex, England, a

British company Application February v21, 1940, Serial No. '320,173 InGreat Britain March 31, 1939 (Cl. Z50-81) 9 Claims.

The invention relates to the production of materials which fluoresceunder the stimulus of X- rays or gamma-rays, to the production offluorescent screens containing such materials and to the production ofphotographicmaterials embodying such fluorescent screens and suitablefor the production of X-ray and gamma-ray photographs, that is so-calledradiographs.

Asis well known, X-rays and gamma-rays from radium and other radioactivematerials are capable of forming latent images in silver halideemulsions and it is possible therefore to make radiographs byinterposing the article to be radiographed between a source of X-rays orgammarays and a silver halide photographic emulsion. l

.It is also well known that the time of exposure can be reduced bymaking use of an intensifying screen which contains a substance, forexample calcium tungstate or zinc sulphide, which fluo- When an rescesunder the influence of X-rays. l intensifying screen is placed incontact with the photographic emulsion and the emulsion and the screenare exposed to X-rays, the fluorescent light emitted produces a latentimage in the emulsion more quickly than the X-rays alone.

In order to reduce the exposure time necessary to obtain a satisfactoryimage, it has been proposed to` combine with a sensitive emulsion on atransparent support a layer of material which ,v fluoresces under theinfluence of X-rays, the

layer being incorporated in or on the emulsion or embodied in thesupport. However, such combinations of fluorescent` andlight-sensitivematerials have not found favour in practice largely because of thedilculty and expense involved in fthe preparation of known fluorescentsubstances which might be so employed.

It has been a growing practice in Irecent years y in industriallradiography `to employ X-rays of short wavelength generated a highkilovoltages, for example 200 to 500 k. v. or alternatively to use thegamma rays from radium or its emanation,

for the radiography off' thick and/or very dense objects, for exampleheavy metal castings.

It is found that` under these conditions, neither zinc sulphide norcalcium, tungstate are wholly satisfactory; zinc sulphide' screens onlyresult in the shortening of the exposure time to asmall degree whilst inthe case of calcium tungsta'te the Iscreens have to be very heavilycoated with calcium tungstate in order to obtain results of any valueand even then the necessary thickness of the screens makes them verycostly, reduces their flexibility and results in images of poordefinition.

Moreover, when used repeatedly in contact with photographic emulsions,the usual costly types of intensifying screens soon become damaged andunfit for further use, particularly if, as is often the case inindustrial radiography, they have to be bent or moulded to the shape ofthe object under test. There is, therefore, a need in the eld ofindustrial radiography fora flexible sensitive material which will giveradiographs free from the blemishes due to t e use of` a damagedintensifying screen, which will give satisfactory results with shorterexposure times than are' necessary if no intensifying screen isemployed, which `will give its optimum intensillcation with thinnerlayers than doesycalcium tungstate andwhich will, at the same time, notbe too costly.

It has been discovered, and this forms the basis of the presentinvention, that crystalline lead sulphate in a state of substantialpurity emits blue-violet light under the influence of X-rays orgamma-rays. With X-rays of the longer wavelengths produced at lowkilovoltages (i, e. 40 to 80 k. v.) it does not iluoresce so strongly asthe known fluorescent substances such as zinc sulphide and calciumtungstate; nevertheless if f prepared as described below, it uorescesstrongly v kilovoltages".

yunder the influence of X-rays particularly at high Accordingly, thepresent invention comprises processes for the production of leadsulphate adapted to fluoresce strongly under the stimulus of X-rays orgamma-rays, fluorescent lead sulphate produced by such processes; X-rayscreens wherein the fluorescent material is substantially purecrystalline lead sulphate, and X-ray sensitive material which comprisesin a single unit,

, one or more X-ray sensitive silver halide emulsion layers,^a supportfor such layers and a. layer of fluorescent lead sulphate.

In accordance with this invention, lead sulphate. material adapted tofluoresce strongly under the stimulus of X-rays orgamma-rays may beprepared by first preparing substantially pure crystalline lead ysulphate and then subjecting this to heat treatment at temperatures ofthe order of 700 to 1000 C. and preferably between 800 to 900 C.Conveniently the heat treatment or flring may be effected by heating thelead sulphate in a crucible lined with calcined calcium sulphate in amuiile furnace.

The optimum length of time for the firing treatment varies with thetemperature of the treatment. Many hours are required at 700 C., aboutfour hours at 800 C., and about 1 to 2 hours at 900-1000" C. Attemperatures below tivity is also detrimental to the character of theproduct. By giving appropriate periods of firing,

.- products may be obtained having an intensification factor exceeding15 and frequently as high as 25 to 30.

The firing of fluorescent materials to increase their fluorescentqualities is common practice and, as is well known, in such firingoperations the fiuorescent materials may be mixed with otherI salts asfluxes before firing. This proce-A dure may also be desired.

The intensification factors given above relate to the use of a singlethick layer of lead sulphate (sufficiently thick to give substantiallythe maximum emission of light from the' surface of the adopted in thepresent case, if

' layer) when used as an intensifying screen placed behind and incontact with a normal type of blue-sensitive X-ray emulsion and exposedto X- rays generated at .1.00 k. v. They are the factors by which theexposure time would have .to be increased in order to give an image ofequal density were no intensifying screen used.

The intensification factor of the lead sulphate depends to some extenton the method employed for the production of the initial substantiallypure crystalline lead sulphate. One convenient method by whichsuch,initialmaterial may be prepared is as follows:

A solution of a pure lead salt (other than lead sulphate) in distilledwater is added to a solu.

tion oi' sulphuric acid, an alkali metal sulphate or another solublecolourless sulphate in distilledI water, or vice versa, in order toproduce and precipitfite lead sulphate by double decomposition. Theinitial lead salt may be a salt of an inorganic acid (and lead nitrateis preferred) or it may be a salt of an organic a'cid, e. g. leadacetate. The lead salt should be asl pure as possible and should lbefree from more than the most minute traces of such metals as iron,copper, nickel, cobalt, chromium and, other metals which yield colouredsulphates or the sulphates of which are unstable at low or hightemperatures.

.Even when the purest commercially available form of the selected leadsalt is used, it is usually advantageous to purify it further. Oneconvenlent method is to precipitate a small fraction of the lead bytreatment of the whole lead salt solution with a small quantity of anammoniacal solution of an alkali metal carbonate or sulphide, lter offthe precipitated lead carbonate or sulphide (which may containimpurities) and then acidify and boil the mother liquor before using itfor the formation of lead sulphate.

The sulphuric acid or other sulphate used to effect the precipitationshould also be free from more than. the most minute traces ofimpurities.

ter precipitation, the lead sulphate is preferwhat according to thepurity of the reagents and the conditions of the precipitation, andrarely exceeds 10.

As indicated above, according to a further feature of this invention,X-ray screens are produced using substantially pure crystalline leadsulphate as the uorescent substance. For this purpose the lead sulphatemay be dispersed in any suitable binding agent, e. g. a solution of acellulose derivative or of a colourless synthetic resin, and theresulting solution cast to form a lm or coated on a paper or othersupport. Gelatin and other hydrophillic colloids may also be used as thevehicle but are not so satisfactory as those soluble in anhydroussolvents. A uorescent screen thus produced may be used in the usualmanner of X-ray intensifying screens; i. e. it may be placed in contactwith an X-ray sensitive silver halide emulsion and the emulsion and thescreen exposed .to X-rays, the fluorescent light emitted by the screenproducing a latent image in the emulsion more rapidly than the lX-raysalone,

In the accompanying drawing similar reference numerals refer to similarparts throughout the several views:

Fig. 1 is a fragmentary, enlarged, cross-sectionalview of an X-rayscreen in accordance with this invention in operative relation with anX-ray sensitive element,

Fig. 2 is a fragmentary, enlarged View of an X-ray sensitive elementhaving a lead sulphate screening layer,

Fig. 3 is a fragmentary, enlarged, cross-sectional view of a modifiedX-ray sensitive element having a lead sulphate screening layer,

Fig. 4 is a fragmentary, enlarged, cross-sectional view of a furthermodified X-ray sensitive element having a lead sulphate screening layerand a thin support,

Fig. 5 is a fragmentary, enlarged, cross-sectional view of an X-raysensitive layer having a removable membrane containing lead sulphate.

The arrangement of Fig. 1 comprises a paper support having a layer 2y ofthe fluorescent lead a sulphate hereof imposed thereon. Itis placed inably washed free from mother liquors by means V operative relationwiththe X-ray lm element composed of a transparent support 3 having X-ray sensitive emulsion layers 4 and 4' imposed on each side thereof.

According to a further feature of this invention indicated above,substantially pure crystal- `line lead sulphate is used to form afluorescent layer in a unitary X-ray sensitive material consisting ofone or more emulsion layers, a support (e. g, of paper or cellulosicfilm) and a fluorescent layer. In such a unit the lead sulphate layeris,preferably situated between the emulsion and its support or isincorporated in the support. This is illustrated in Fig. 2 wherein atransparent support 3 bears a lead sulphate' layer 2 on which is imposedsilver halide layer 4.

Other 'arrangements are possible, but it is necessary to take intoaccount the opacity of the lead sulphate layer both to visible light andto X-rays and gamma-rays. For example, the lead sulphate layer may be onthe face of the emulsion provided the support is transparent to visiblelight as well as X-rays and the emulsion is exposed to rays passingthrough the support. Such an element is shown in Fig. 3 and comprises atransparent support 3 bearing a silver halide layer 4 on which isimposed lead sulphate layer 2. Further, if the support is very thin andtransparent to visible light and X-rays, the lead sulcasacca back of thesupport, it may be suspended in a 4 vehicle soluble in water or dilutealkali, for example, sh glue, or an acid resin, which would permit theready removal of the lead sulphate layer before or during processing ofthe emulsion. Again, the lead sulphate layer may be of such a characterthat it can be stripped, in the wet or dry condition, from the assembly.A layer of lead sulphate in a vehicle such as iish glue, or in astrippable material, e. g. collodion, may in consequence be coated on tothe surface of the emulsion remote from the support,'and the layer maybe removed before processing so that the finished radiograph can beviewed by reflected light if the support is not transparent or bytransmitted light if the support isV transparent. Such an element isshown in Fig. 5 and comprises a transparent base 3 having imposedthereon a silver halide layer 4 which carries a collodion membrane 5having distributed therethrough lead sulphate.

Owing to the insolubility of lead sulphate in the usual photographicdeveloping and xing baths, unless it is coated on the surface andremoved in the manner described above, the lead salt remains in thefinished radiograph and although this is a disadvantage for somepurposes, it is of no importance where the support is opaque, forexample of paper, and is viewed by reflected light.

X-ray sensitive material embodying a fluorescent lead sulphate layer inaccordance with the invention may be produced by dispersing thefluorescent lead sulphate in a colloid vehicle, for example a -cellulosederivative,` a colourless synthetic resin, a solution lof gelatin or thelike,

coating the resulting dispersion on a support by any of the usualcoating methods employed in the photographic industry, drying theproduct and then coating a gelatino-silver halide emulsion of the typecommonly employed for radiographic purposes on the top of` the leadsulphate layer to form a unitary product. When a waterpermeable vehiclesuch as gelatin 'is employed for the leadV salt, it is preferable toapply a protective coating, for examp1eof ccllodion or other varnish, tothe lead sulphate layer before applyling the gelatino-silver halideemulsion in order to avoid any possibility of interaction between theemulsion and the lead salt or between the materials of processing baths'and the lead salt, i

as lead sulphate is not completely insoluble in water.

Lead sulphate material adapted to iluoresce strongly under thestimulusof X-rays or gammajrays possesses many advantages as the activefluorescent material in an X-ray screen. Thus it is relatively cheap(compared, for example, with calcium tungstate), only a thin layer of itis necessary with the consequential advantages that ilexible X-rayscreens containing it are easily producible and, ysince it can readilybe obtained in micro crystalline form, screens containing it as theactive ingredient result in radiographs of very goed dcilnition.

1 claim: A

1. An X-ray screen comprising a support and a layer which fluorescesunder the influence of X-rays comprising substantially pure crystallinelead sulphate obtainable byA heating a substantially pure lead sulphateat a temperature of 700 C, to 1000c C. for a period of at least onehour.

2. An X-ray sensitive element for radiography which 'comprises an X-raysensitive emulsion, a support for the emulsion and a screen layercontaining substantially pure crystalline lead sulphate obtainable byheating a substantially pure lead sulphate at a temperature of 700 C. to1000o C. i'or a period of at least one hour which will fluoresce underthe stimulus of X-rays or gamma-rays and so intensify the effect of therays on the sensitive emulsion, the whole combined to form a unitaryproduct.

3. An X-ray sensitive element according to claim 2 wherein the leadsulphate screen layer is capable of ready removal from' the assembly iafter the emulsion has been exposed.

4. Process for the production of lead sulphate material adapted toluoresce strongly under the stimulus of X-rays and gamma-rays whichcornprises heating substantially pure crystalline lead sulphate at atemperature'of 700 to 1000 C. for

V a period of at least about 1 hour.

5. Process for the production of lead sulphate material adapted touoresce strongly under the stimulus of X-rays and gamma-rays whichcomprises .heating substantially pure crystalline lead sulphate at atemperature of 800 to 000 C.

6. Process for the production of lead sulphate material adapted tofluoresce strongly under'the stimulus of X-rays and gamma-rays whichcomprises forming substantially pure crystalline lead' tion factorexceeding 15 and obtainable by heat- `ing substantially pure leadsulphate at a temperature of 700 to 1000" C. for a period of at leastabout one hour.

8. An X-ray screen comprising a support and a layer of nely dividedsubstantially pure lead sulphate `uniformly dispersed in a bindingmedium, said lead sulphate having been fired at a Y temperature from 700to 1000 C. for a period of at least one hour. l

9. An X-ray screen comprising a support and a layer of finely dividedsubstantially pure lead sulphate uniformly dispersed in a bindingmedium, said lead sulphate being substantially identical with thatobtainable by firing substantially pure lead sulphate at a temperaturefrom 800 to 900 C. for a period of at leastl one hour.

FRANK FORSTER RENWICK.

